Several distinct minor polypeptides have been observed from lysates of highly purified SV40 virus including the protein in SV40 DNA-protein complex. The molecular weights range from 21 K to 120 K daltons. Of these several are antigenically related to Vp1 with molecular weights larger than that of Vp1. Our goals are (1) to study the nature and function of these minor polypeptides in the virus and their relation to viral polypeptides by biochemical and immunological methods and by the use of deletion mutants of Simian Virus 40 and (2) to study the process of the polypeptides synthesis during infection.